In recent years, there are increasing demands for reductions in the size and the weight of elements by making the surface and the inside of a structure (components), which is formed from transparent materials such as glasses and plastics, highly sophisticated, and further by incorporating such components. For meeting such demands, two approaches are taken. One is a technical approach on the material side, i.e., making the materials themselves into composites, hybrids, or the like, and another is a technical approach on the processing side, i.e., incorporating functional regions or performing structure control.
Specifically, as a technical approach on the processing side, processing of the material surface is performed by techniques such as polishing, grinding, dry etching, and wet etching. However, when making a complex surface structure, since the number of processes increases and the processed regions are restricted to two-dimensional processing of the material surface, the degree of freedom in processing is low. Moreover, since gases and liquids are discharged together with processing waste after processing, it is required to treat them appropriately from an environmental viewpoint. On the other hand, as a method (technology) for forming, inside the structure formed from a transparent material, a structure region, which has a different material quality from that of the original material, phase separation (composition change) and a method for causing crystallization by use of external fields, such as heat, pressure, an electric field, a magnetic field, and an optical electric field, have been examined. However, apart from the optical electric field, the other external fields are unsuitable for processing which forms different structures at arbitrary places (regions) inside the structure since they affect the entire structure.
On the other hand, there is a method to form a permanent refractive index-changing region inside a structure by irradiating a transparent material with a condensed laser beam with an extremely short pulse width of 10×10−12 seconds or less, as the optical electric field. This processing method is able to form complex shapes three-dimensionally at arbitrary places inside the structure and the lamination of structure is also easy. Furthermore, it is attracting attention since there is no release of processing waste and thus, the environmental burden is also low. For example, various structures which form diffraction gratings, internal structures with optical functionality such as photonic crystals, or micro flow paths, have been proposed (for example, see patent documents 1-5 and non-patent documents 1-8).
Since it becomes possible to highly sophisticate these structures having a light-controlling function such as diffraction gratings or photonic crystals, as differences in refractive indices between that of a transparent material and that of a refractive-index changing region increase, selection of such transparent materials, methods for irradiating lasers, or the like is studied.
The refractive-index changing region is formed by various causes such as densification, presence of cavities, phase splitting, crystallization, changes in valence, and the like and the refractive index also varies depending on the combination of transparent materials subjected to laser irradiation and irradiating conditions. Plastics or glasses are used as transparent materials in many cases for their satisfactory formability and processability. However, since plastics generally have lower refractive indices compared to those of inorganic materials such as glass and thus, inferior in heat-, water-, and chemical resistances or the like, there are many limitations when they are used as optical components.
For this reason, many inorganic glasses are used for the structures processed by ultrashort pulse lasers. Moreover, many of the refractive index-changing regions formed inside glass are caused by densification and variation of the refractive index thereof is approximately 0.1 to 1%.
Accordingly, in the below non-patent document 1, in order to achieve much larger differences in refractive indices, optical waveguides or photonic crystal structures which have a high refractive-index region where crystallites of a compound semiconductor are deposited/grown by irradiating the inside of a mother glass in which compound semiconductor is dispersed with an ultrashort pulse laser beam, is proposed. Specifically, a three-dimensional photonic crystal structure (log-pile structure) which uses a region where the refractive index is high and where the concentration of fine particles of CdSe crystals is high only in focal region is disclosed. However, in this case, it is not preferable since the mother glass needs to contain components such as Cd and Se, which poses a heavy burden on the environment.
On the other hand, optical components forming a cavity inside a glass and using the difference in the refractive indices of this cavity (a part with reduced refractive index) and of a part which is not laser-irradiated yet has been proposed. Although the mechanism which forms cavities inside the glass of these optical components is not yet clear at this stage, it is considered as following. When a laser beam with an extremely short pulse width and an extremely high field strength per unit time and unit space, such as a femtoseconds pulse laser beam, is condensed and irradiated on the inside of transparent materials, numerous free carriers generate within this extremely short time through non-linear optical effects such as a multiphoton absorption process or a tunnel effect. Atoms (nuclei) from which electrons are stripped off are positively charged and cause Coulomb explosion due to repulsion among positive charges. Nuclei which are present at the place are spread around by this explosion and remain fixed to form cavities.
As examples of structures having such cavities internally, below non-patent document 2 discloses a two-dimensional photonic crystal structure in which a fine cavity tube is arranged in a triangular lattice-like manner inside a silica glass; below non-patent document 3 discloses a three-dimensional photonic crystal structure where cavities inside the silica glass, which is doped with 10% Ge, are laminated in a face-centered cubic lattice-like manner; the below patent document 3 discloses optical attenuating waveguide material using cavity section which is formed in optical fiber made of silica glass; and the below non-patent document 4 discloses reading-out of optical memory using the cavity inside silica glass.
Although it is known that the abovementioned cavity is formed in the aforementioned silica glass, a Ge-doped silica glass, or Corning 0211 (zinc borosilicate glass), which is described in below non-patent document 8, it has not been clarified specifically what kind of composition is required for a glass to form an internal cavity. For this reason, currently available structures (components), which have an internal cavity, have not reached the stage where optical properties thereof such as refractive index or other physical properties such as thermal-, mechanical-, or electrical properties can be selected, and the degree of freedom in component design is also low. Moreover, a silica glass or a Ge-doped silica glass has a considerably high melting temperature and thus, a considerably high working temperature is required in order to obtain such glasses. For this reason, there are also problems of high costs for energy and the need for special manufacturing methods.    [patent document 1] Japanese Laid-Open Patent Application No. 2002-311277    [patent document 2] Japanese Laid-Open Patent Application No. 2003-506731    [patent document 3] Japanese Laid-Open Patent Application No. 2004-279957    [patent document 4] Japanese Laid-Open Patent Application No. 2003-236928    [patent document 5] Japanese Laid-Open Patent Application No. 2004-196585    [patent document 6] Japanese Laid-Open Patent Application No. 2003-260581    [non-patent document 1] N. Takeshima, Y Narita, T. Osada, S. Tanaka, K. Hirao“Three-dimensional micromachining of glass by femtosecond laser” 45th symposium on glass and photonics material (National Institute for Materials Science, Advanced Materials Laboratory, Tsukuba, Ibaraki) A-1 (lecture abstract pp. 2-3)    [non-patent document 2] H-B. Sun, Y Xu, S. Matsuo and H. Misawa, Optical Review Vol. 1.6, No. 5 (1999) pp. 396-398.    [non-patent document 3] H-B. Sun, Y Xu, K. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, Opt. Lett., 26 (2001) pp. 325    [non-patent document 4] M. Watanabe, S. Juodkazis, H-B. Sun, S. Matsuo, H. Misawa “Transmission and photoluminescence image of three-dimensional memory in vitreous silica” Applied Physics Lett. Vol. 74, No. 26 (1999) pp. 3957-3959.    [non-patent document 5] V. Mizeikis, K. Yamasaki, S. Juodkazis, S. Matsuo, H. Misawa, “Laser microfabricated photonic crystal structure in PMMA” The 50th Meeting of the Japan Society of Applied Physics (Kanagawa University, Yokohama, Kanagawa) 27p-YN-7 (lecture proceedings, issue 3, pp. 1124)    [non-patent document 6] K. Yamasaki, M. Watanabe, S. Juodkazis, S. Matsuo, H. Misawa, “Three-dimensional processing of polymer film by irradiation of condensed femtosecond laser pulse” The 49th Meeting of the Japan Society of Applied Physics (Tokai University, Hiratsuka, Kanagawa) 28p-YC-9 (lecture proceedings, issue 3, pp. 1119)    [non-patent document 7] K. Yamasaki, S. Juodkazis, S. Matsuo, H. Misawa, “Three-dimensional micro-channels in polymers: one-step fabrication” Applied Physics A, Vol. 77, No. 3-4, pp. 371-373 (2003)    [non-patent document 8] C. B. Schaffer, A. O. Jamison, E. Mazur “Morphology of femtosecond laser-induced structural changes in bulk transparent materials” Applied Physics Lett. Vol. 84, No. 9 (2004) pp. 1441-1443    [non-patent document 9] K. Yamada, W. Watanabe, Y. Li, K. Itoh, J. Nishii, “Multilevel approximation of phase-type diffractive lens in silica glass induced by filamentation of femtosecond laser pulses” Opt. Lett. Vol. 29, No. 16, pp. 1846-1848 (2004)    [non-patent document 10] S. Matsuo, H. Misawa, “Direct Measurement of laser power through a high numerical aperture oil immersion objective lens using a solid immersion lens” Review of Scientific Instrument, Vol. 73, No. 5 (2002) pp. 2011-2015.